Develop algorithm to calculate skymap overlap involving UNIQ sky maps

Gwcelery is moving to a system of focusing primarily on multi-order sky maps, i.e. non "flat" sky maps. These have increased resolution around higher regions of probability density, as opposed to equal area pixels that "flat" (NESTED or RING) sky maps have.

The main case will be finding a way to combine a UNIQ with a flat skymap, although a general method that works for two UNIQ sky maps would be interesting as well.

More documentation on multi-order sky maps can be found here: https://igwn.readthedocs.io/projects/userguide/en/stable/tutorial/multiorder_skymaps.html